The trick that I used on the Sun Worm was using Beast Form to jump up on one of the spiky ledges on the ceiling, switching to Nature Form in midair, and then switching to Energy once you're clinging to the ledge. If you switch to Energy after you're clinging on, you shouldn't take damage. Be ready to switch back to Nature pretty quick, though, cause you will occasionally bounce up and take damage when you fall back. Have some healing items ready. But once you're latched on, you can pretty much just spam energy form shots down into the water. It helps if you have Li, too.